High speed modulation of switched-focus x-ray tube

ABSTRACT

A dose-modulated irradiating system includes an x-ray tube ( 20 ) with at least a filament ( 80 ) for generating electrons, a cathode ( 84 ) and an anode ( 92 ) for accelerating and collimating the generated electrons into an electron beam ( 94 ), and an electrostatic grid with grid electrodes ( 110, 112 ) for steering the electron beam ( 94 ) on the anode ( 92 ). The anode ( 92 ) generates an x-ray beam ( 96 ) responsive to the electron beam ( 94 ). Grid biasing is provided for applying a time-varying electrical bias to the grid electrodes ( 110, 112 ) that produces a first time-varying intensity modulation of the electron beam ( 94 ). A current of the filament ( 80 ) is modulated to produce a second time-varying intensity modulation of the electron beam ( 94 ). A controller ( 52 ) controls cooperatively combining the first and second time-varying intensity modulations to produce a combined time-varying intensity modulation.

The following relates to the diagnostic imaging arts. It findsparticular application in conjunction with dose-modulation by cascadingfrequency responses of x-ray tube filament current and grid voltage forcomputed tomography imaging, and will be described with particularreference thereto. However, the following relates more generally todose-modulated and other types of computed tomography imaging and tox-ray tubes used in other applications.

In a typical computed tomography imaging apparatus, an x-ray tube ismounted on a rotating gantry that defines an examination region insidewhich an imaging subject is disposed. The x-ray tube rotates about thesubject on the rotating gantry and projects a wedge-, fan-, conical-, orotherwise-shaped x-ray beam through the examination region. Atwo-dimensional x-ray detector disposed on the rotating gantry acrossthe examination region from the x-ray tube receives x-ray beam afterpassing through the examination region. Suitable electronics estimatex-ray absorption data based on the detected x-ray intensities, and animage reconstruction processor reconstructs an image representationbased on the absorption data.

The x-ray tube in the above arrangement typically includes a filamentthat generates electrons. A cathode cup partially surrounds the filamentand is biased negatively to focus the electrons into an electron beam. Abias in the kilovolt range between the cup and a rotating anodeaccelerates the electron beam to the anode, causing it to emit x-rays.

A problem arises in that the imaging data is under-sampled in therotational direction. Sampling occurs at a spatial frequency related tothe detector spacing, while sampling theory calls for a doubled spatialfrequency to avoid aliasing and other sampling-related artifacts. Tocounteract under-sampling, an x-ray tube is employed in which the focalspot is alternated or wobbled between two discrete positions in therotational direction between measurements to spatially interleavesamples. Alternatively, a quarter ray offset can be employed in whichrays from opposing 180° projection views are spatially interleaved.

To effect beam wobble, the x-ray tube includes grid electrodes arrangedon opposite sides of the filament. The grid electrodes are biased withan alternating polarity to generate a switched electrostatic forceorthogonal to the electron beam that shifts the electron beam betweentwo paths corresponding to the two focal spots of the wobble.Alternatively, an orthogonal electromagnetic force can be used toswitchingly steer the beam.

Another concern in computed tomography imaging is limiting x-rayexposure of the subject. In medical imaging applications, the x-raydosage delivered to the patient is a regulated safety parameter. Inairport security scanning, the x-ray intensity is advantageouslyadjusted to account for differing x-ray absorption characteristics ofdifferent types of luggage. Various dose-modulated computed tomographytechniques have been developed. However, these past methods have certaindisadvantages.

In one method, described in U.S. Pat. No. 5,867,555 issued to Popescu etal., dose modulation as a function of angular position is obtained bysynchronously modulating the x-ray tube filament temperature. As notedin that reference, however, the modulation index obtained by filamentcurrent control is limited by a cooling rate (thermal mass) of thefilament, especially at higher rotation speeds. The Popescu techniquechanges the dose once every 180° in a scanner that rotates once every0.75-2.0 seconds, which pushes the limit of the filaments ability torespond. Thermal mass limitations are also exaggerated for higherdynamic modulation ranges which involve substantial cooling of thefilament. The cooling rate decreases as the temperature decreases.

Modern scanners rotate twice per second and faster speeds are startingto appear in product. Moreover, more dose changes per revolution areadvantageous, setting modulation speeds that exceed the physical limitsof the Popescu technique.

In another known method, a Wehnelt cylinder incorporated into the x-raytube selectively pinches off the beam to switch the beam on and off.Beam switching is synchronized with a CINE frame sequence todose-modulate by controlling a duty cycle of the x-ray beam. However,this dose modulation approach reduces the amount of collected angulardata.

The present invention contemplates an improved apparatus and method thatovercomes the aforementioned limitations and others.

According to one aspect, a dose-modulated irradiating system for anx-ray tube is disclosed. The x-ray tube has a cathode including afilament that generates electrons which are focused into a beam, and ananode that generates x-rays responsive to the electron beam. At leastone electrostatic control electrode is arranged to electrostaticallyreduce an intensity of the electron beam. A biasing means is providedfor applying a time-varying electrical bias to the electrostatic controlelectrode to vary the intensity of the electron beam.

According to another aspect, a method is provided for dose-modulating anoutput of an x-ray tube. The x-ray tube includes a cathode having afilament that generates electrons which are focused into a beam, ananode that generates x-rays responsive to the electron beam, and anelectrostatic control electrode that electrostatically adjusts anintensity of the electron beam. A time-varying electrical bias isapplied to the electrostatic control electrode to produce a firsttime-varying intensity modulation of the electron beam.

One advantage resides in providing a high dynamic range of dosemodulation at a high modulation frequency. This facilitates dosemodulation using present- and next-generation computed tomographyscanners which rotate at rates of 120 rpm or higher.

Another advantage resides in providing such dose modulation withoutmodifying the tube design.

Yet another advantage resides in providing such dose modulation withoutadditional high speed electronics.

Numerous additional advantages and benefits will become apparent tothose of ordinary skill in the art upon reading the following detaileddescription of the preferred embodiments.

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for the purpose ofillustrating preferred embodiments and are not to be construed aslimiting the invention.

FIG. 1 shows a dose modulated computed tomography imaging apparatus.

FIG. 2 schematically shows a suitable dose-modulated irradiating systememploying electrostatic beam wobbling.

FIG. 3 shows schematically shows a suitable dose-modulated irradiatingsystem employing electromagnetic deflection beam wobbling.

With reference to FIG. 1, a computed tomography imaging scanner 10includes a subject support 12 for moving a subject such as a medicalpatient, an item of luggage undergoing a security scan, or the like intoor within an examination region 14 defined by a rotating gantry 16. Anx-ray tube 20 arranged on the gantry 16 transmits a fan-shaped,wedge-shaped, conically-shaped, or otherwise-shaped x-ray beam into theexamination region 14. A two-dimensional x-ray detector 22 disposed onthe gantry 16 across the examination region 14 from the x-ray tube 20measures a spatially-varying intensity of the x-ray beam after the x-raybeam passes through the examination region 14. Typically, the x-raydetector 22 is mounted on the rotating gantry 16. In another suitablearrangement, the detector is arranged circumferentially on a stationarygantry surrounding the rotating gantry. Gantry rotation is controlled bya gantry rotation controller 24.

In helical computed tomography imaging, the gantry 16 rotatessimultaneously with a linear motion of the subject support 12 to effecta helical trajectory of the x-ray tube 20 about the examination region14. In axial computed tomography imaging, the gantry 16 rotates whilethe subject support 12 remains stationary to effect a circulartrajectory of the x-ray tube 20 about the examination region 14. Involumetric axial imaging, the subject support 12 is repeatedly steppedlinearly with an axial scan performed for each step to acquire multipleimage slices along the axial direction.

Acquired imaging projection data with an index of the apex of the fan orcone and of the trajectory within the fan or cone is transmitted off thegantry 16 and stored in a digital data memory 30. A reconstructionprocessor 32 reconstructs the acquired projection data, using filteredbackprojection or another reconstruction method, to generate athree-dimensional image representation of the subject or of a selectedportion thereof, which is stored in an image memory 34. The imagerepresentation is rendered or otherwise manipulated by a video processor36 to produce a human-viewable image that is displayed on a graphicaluser interface 38 or another display device, printing device, or thelike for viewing by an operator. Preferably, the graphical userinterface 38 is programmed to interface a radiologist with the computedtomography scanner 10 to allow the radiologist to execute and controlcomputed tomographic imaging sessions.

During imaging, it is advantageous to modulate a x-ray radiation dosagereceived by the subject. For non-cylindrical objects, it is advantageousto have a higher x-ray beam intensity along the major axis and a lesserintensity along the minor axis. For example, in rapid airport securityscanning of luggage the x-ray beam intensity is preferably adjusteddepending upon the x-ray absorption characteristics of the scannedluggage. In medical imaging applications, the intensity is preferablyadjusted to maintain a constant radiation dose for each axial slice oreach helical rotation as an imaging scan passes through regions of thebody having differing x-ray absorption densities. Present computedtomography scanners provide rotation rates of about 120-150 rpm, androtation rates of 180-240 rpm and higher are contemplated in theforeseeable future.

A 120 rpm rotation rate corresponds to an acquisition time of 0.25seconds for a 180° angular span of data, while a 180 rpm rotation ratecorresponds to an acquisition time of 0.167 seconds for 180° of data.Hence, the dose modulation should have a frequency response of about 4Hz or higher for a 120 rpm gantry rotation rate, and should have afrequency response of about 6 Hz or higher for a 180 rpm gantry rotationrate, to provide dose modulation that tracks the gantry rotation. Higherrotation speeds produce proportionately higher anode milliamperemodulation rates. For medical imaging applications, a dose modulationdynamic range of at least about 8:1 to 10:1 is desired. That is, a ratioof a maximum x-ray beam intensity to a minimum x-ray beam intensityduring the modulation at all rotation speeds should preferably be atleast about 8:1 to 10:1.

To effect dose modulation conforming to these specifications, a dosedelivery rate processor 50 integrates the x-ray absorption data over anarea of the x-ray detector 22 and inputs a continuously updated dosedelivery rate to a dose modulation controller 52. The gantry rotationcontroller 24 also inputs gantry angular position to the dose modulationcontroller 52 for optional synchronization of the dose modulation withangular position of the x-ray source 20.

With continuing reference to FIG. 1, the dose modulation controller 52outputs two synchronized, cooperating dose modulation control signals: afilament modulation signal 60 directed toward an x-ray tube filamentpower supply 62, and a grid modulation signal 64 directed toward a gridpower supply 66. The grid power supply 66 additionally receives a beamwobble input signal 70 which alternates or wobbles a focal spot of thex-ray tube 20 to counteract under-sampling and aliasing in the radialdirection. The x-ray tube 20 is driven and controlled by outputs of thefilament power supply 62, the grid power supply 66, and a tube powersupply 72 which sets the potential between the cathode and the anode.

With continuing reference to FIG. 1 and with further reference to FIG.2, which schematically shows components of the x-ray tube 20 incross-section along with associated electronic components, an elongatedfilament 80, preferably having a wire diameter, for a conventionalcoiled wire filament, of 8-10 mil, is arranged in a cathode cup 82 of acathode 84. The filament 80 generates electrons by thermionic emission,field emission, or another mechanism in response to a filament currentinput delivered by the filament power supply 62. Preferably, anisolation transformer 86 and insulating standoffs (not shown)electrically isolate the filament 80 from other elements of the x-raytube 20. Typically, the filament current is a pulse-width modulatedcurrent with a frequency of about 5 kHz to about 30 kHz and an amplituderanging from a few hundred milliamperes to a few amperes.

With reference to an electrical common 90, the cathode 84 is typicallybiased negatively at a bias of around −40 kV to −70 kV and the anode to+40 kV to +70 kV for a bipolarly biased tube. The cathode cup 82 isshaped such that the negative charge defines an electrostatic electricfield that collimates electrons generated by the filament 80. Theelectrons of the electron beam accelerate across the cathode-to-anodegap, which is typically about 2 cm.

In response to the accelerated electron beam 94 striking the anode 92,the anode emits an x-ray beam 96 that passes out of a vacuum volume (notshown) containing the filament 80, the cathode 84, and the anode 92. Theanode 92 rotates about an axis 100 and includes an angledcircumferential x-ray generating surface 102 that interacts with theelectron beam 94 as the anode 92 rotates. The rotation distributes heatacross the circumferential surface 102.

With continuing reference to FIGS. 1 and 2, the cathode cup of the x-raytube 20 further includes first and second grid electrodes 110, 112arranged on opposite sides of the filament 80 such that the electronbeam 94 passes between the grid electrodes 110, 112. The grid electrodes110, 112 are used to electrostatically modify the electron beam 94.

Specifically, biasing both electrodes 110, 112 more negative narrows awidth of the electron beam 94 by electrostatic constriction oraperturing, which affects the focus at the anode. A relatively largernegative sum voltage applied to both electrodes 110, 112 further reducesan intensity of the electron beam 94 at the anode 92 by partialelectrostatic pinchoff of the electron beam 94, i.e. reduces the tubecurrent. For a sufficiently large negative bias voltage applied to bothelectrodes 110, 112 the electron beam 94 can be pinched off entirely.

Additionally, an applied voltage difference, imposed upon the voltagesum, and between the grid electrodes 110, 112, effects a beamdeflection.

In another embodiment, the voltage to each grid may be produced as thesum of individual voltages for constriction and deflection, such thatthe focal spot or spots may be jointly moved essentially tangentially tothe rotating anode surface while maintaining correct spot to spotdisplacement and width.

Additional elements such as one or more additional filaments forredundancy or for achieving different electron beam characteristics andthe like, (elements not shown) are also optionally included in thecathode assembly.

With continuing reference to FIGS. 1 and 2, the filament power supply 62is controlled by the filament modulation signal 60 produced by the dosemodulation controller 52. Modulation of the filament current causesmodulation of the filament temperature and corresponding modulation ofthe electron generation rate. This, in turn, modulates the intensity ofthe electron beam 94 for dose modulation. Changing the filament currentdoes not significantly modify electron trajectories and so does notgreatly alter the focus of the electron beam or change the size of thefocal spot. However, the filament 80 has a thermal mass which limits thefrequency response of this modulation control. For a preferred filamenthaving a 200 micron (8 mil) diameter, modulating the filament current at4 Hz achieves the frequency response of dose modulation of 4 Hz withcorresponding dynamic range of about 2:1 to 3:1. A larger diameterfilament has more thermal mass and will typically exhibit a slowerfrequency response and reduced dynamic range.

In order to obtain dose modulation with better frequency response thanthat achieved using filament control alone, the dose modulationcontroller 52 additionally outputs the grid modulation signal 64directed toward a grid power supply 66. The grid modulation signal 64 iscombined with the beam wobble input signal 70 which alternates orwobbles a focal spot of the x-ray tube 20 to produce combined gridelectrode biasing potentials.

Specifically, a first summer 120 additively combines the grid modulationsignal 64 and the beam wobble input signal 70 to produce a first controlsignal that controls the electrostatic potential that is applied to thefirst grid electrode 110. A second summer 122 subtractively combines thegrid modulation signal 64 and the beam wobble input signal 70 to producea second control signal that controls the electrostatic potential thatis applied to the second grid electrode 112. The grid electrodestherefore receive a differential potential controlled by the beam wobbleinput signal 70 which controls the beam wobble, and a superimposed sumpotential controlled by the grid modulation signal 64.

The frequency response of analog dose modulation performed by modulatingthe sum grid potential is limited by a frequency response of the gridpower supply 66. Existing grid power supplies used for electrostaticbeam wobble alternately position the focal spot between two tightlycontrolled locations using a differential potential applied to the gridelectrodes 110, 112. The beam is stepped between the focal spots inresponse to a square voltage pulse beam wobble input signal 70, ataround 2-4 kHz for a 120 rpm gantry rotation and 2000 projection viewsper rotation.

The harmonic content of the square-wave beam wobble input signal 70leads to high bandwidth requirements for the grid power supply 66, whichis usually a switching power supply. Such power supplies today typicallyexhibit a limited analog frequency response (as opposed to a poweramplifier, which would have a much higher bandwidth but would also bemore costly) such that modulation of the grid potentials alone does notachieve dose modulation at the desired frequency response of about 4Hzto 6 Hz or higher with a dose modulation dynamic range of about 8:1 to10:1.

To achieve the desired dose modulation frequency response and dynamicrange, the dose modulation controller 52 S cooperatively controls thegrid modulation signal 64 and the filament modulation signal 60 tocascade the grid electrode dose modulation and the filament current dosemodulation. For a 4-6 Hz modulation frequency, a product of the cascadedgrid and filament dose modulations provides the desired 8:1 to 10:1dynamic modulation range. Moreover, dynamic range can be traded off forhigher dose modulation frequency.

A further benefit of combined grid and filament dose modulation isimproved focus of the electron beam 94 throughout the dynamic modulationrange. The x-ray tube 20 is designed to operate with a small spot sizeat relatively high output power. Combined grid and filament dosemodulation reduces the output power by cooperatively reducing thefilament current and increasing the sum potential on the grid electrodes110, 112. Beam narrowing due to higher sum grid potentials, results inimproved electron beam spot size on the anode 92 during dose modulationand consequently improved spatial definition of the x-ray beam 96throughout the dynamic modulation range.

In FIG. 2, the first and second summers 120, 122 are integrated into thegrid power supply 66. However, it is also contemplated to arrange theseelements outside of the grid power supply, and supply the summer outputsas inputs to the grid power supply. This alternate arrangementfacilitates retrofitting a computed tomography imaging system whichinclude a grid power supply for beam wobble with dose modulationcapability that uses cascaded filament current and grid sum potentialmodulations.

Moreover, it will be recognized that the cascaded dose modulation can beperformed with or without concurrent focus switching. That is, the beamwobble input signal 70 can be turned off or omitted entirely from thecomputed tomography imaging system while retaining the cascaded dosemodulation aspect.

With reference to FIG. 1, the above-described beam modulation can beemployed in a variety of ways. In one suitable dose modulated imagingprocess, the dose modulation controller 52 integrates the dose deliveryrate received from the dose delivery rate processor 50 over a 360°revolution of the gantry 16 corresponding to an axial slice or one turnof a helical trajectory. Based on the integrated dose delivered to thesubject over 360°, the dose modulation controller 52 adjusts the sumpotential on the grid electrodes 110, 112 and the current through thefilament 80 for the next 360° revolution to maintain a generallyconstant radiation dose delivered for each slice or helical revolution.As the imaging progresses axially between regions of the subject havingdiffering absorption characteristics, the x-ray beam is modulated tomaintain a generally constant x-ray absorption rate in the subject.

In another suitable dose modulated imaging process, the dose modulationcontroller 52 continuously adjusts the sum potential on the gridelectrodes 110, 112 and the current through the filament 80 to maintaina generally constant dose delivery rate as reported by the dose deliveryrate processor 50. This process can adjust the radiation delivery rateduring acquisition of projection data for a single slice, and isparticularly suitable for large-pitch helical scanning in which asubstantial axial distance is sampled during each 360° gantry rotation.

The x-ray tube 20 uses electrostatic beam deflection to implement beamwobble. Hence, the grid electrodes 110, 112 are used for theelectrostatic beam wobble and are additionally used forelectrostatically dose modulating through the applied sum potential.However, in some existing x-ray tubes an electrostatic grid is not used.Rather, the electrostatic grid is replaced by an electromagnetic beamdeflector.

With reference to FIG. 3, an x-ray tube 20′ uses separateelectromagnetic beam deflection for wobble and electrostatic electronbeam constriction or aperturing for dose modulation. FIG. 3schematically shows components of the x-ray tube 20′ in cross-sectionalong with associated electronic components. Components in FIG. 3 thatgenerally correspond with components of FIG. 2 are indicated bycorresponding primed reference numbers.

Specifically, the x-ray tube 20′ includes a filament 80′, a cathode 84′with a cathode cup 82′, and an anode 92′ with a rotational axis 100′ andan angled circumferential x-ray generating surface 102′. Thesecomponents generally correspond to the filament 80, cathode 84, cathodecup 82, anode 92, rotational axis 100, and surface 102 of the x-ray tube20. Moreover, x-ray tube control hardware including a filament powersupply 62′ with an isolation transformer 86′ and a tube power supply 72′operate similarly to corresponding elements 62, 86, 72 of FIG. 2 inapplying bias potentials relative to a common 90′ and generating anelectron beam 94′ and an x-ray beam 96′. A beam wobble input signal 70′and a filament current modulation control signal 60′ correspond to theanalogous input signals 70, 60 of FIGS. 1 and 2.

The x-ray tube 20′ employs electromagnetic beam deflection rather thanthe electrostatic beam deflection employed by the x-ray tube 20.Specifically, the beam wobble input signal 70′ is input into a deflectorpower supply 120. Inverting and non-inverting paths 122, 124 generatepower for driving electromagnets 126, 128 based on the input signal 70′to steer the electron beam 94′ between two wobble position focal spots.Two electromagnets 126, 128 are shown. However, a single solenoidalelectromagnet can also be employed.

Unlike the electrostatic grid electrodes 110, 112 of the x-ray tube 20,the electromagnets 126, 128 of the x-ray tube 20′ cannot constrict theelectron beam to effect a dose modulation. Rather, separateelectrostatic beam constriction elements are included in the tube 20′.

A Wehnelt cylinder 140 is driven by a Wehnelt power supply 142. The gridmodulation input signal 64 of FIGS. 1 and 2 is replaced by an analogouselectrostatic modulation signal 64′ which is input to the Wehnelt powersupply 142. A negative potential applied to the Wehnelt cylinder 140effects a reduction in an intensity of the electron beam 94′ at theanode 92′ by partial electrostatic pinchoff of the electron beam 94′.For a sufficiently large negative potential applied to the Wehneltcylinder 140 the electron beam 94′ can be pinched off entirely. TheWehnelt cylinder 140 is biased negatively with respect to the cathode84′, typically in a range of zero volts to about 3 kV more negative thanthe cathode 84′.

The Wehnelt cylinder 140 performs electrostatic beam modulation based onthe electrostatic modulation signal 64′ in substantially the same mannerthat the grid electrodes 110, 112 of the x-ray tube 20 performelectrostatic beam modulation based on the sum electrostatic potential64. Electrostatic dose modulation via the Wehnelt cylinder 140 issuitably combined with modulation of current in the filament 80′ toprovide cascaded electrostatic and filament current controlled dosemodulation. Benefits of this cascaded dose modulation described withreference to the x-ray tube 20 apply also to the x-ray tube 20′, such asimproved dynamic range of the radiation modulation and improved spatialdefinition of the x-ray beam over the dynamic modulation range.Moreover, the x-ray tube 20′ optionally includes additional elementssuch as fluid passages for active cooling, one or more additionalfilaments for redundancy or for achieving different electron beamcharacteristics, and the like (elements not shown).

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A dose-modulated irradiating system for an x-ray tube with a cathodeincluding a filament that generates electrons which are focused into abeam and an anode that generates x-rays responsive to the electron beam,the dose-modulated irradiating system further including: at least oneelectrostatic control electrode arranged to electrostatically reduce anintensity of the electron beam; and a biasing means for applying atime-varying electrical bias to the electrostatic control electrode tovary the intensity of the electron beam.
 2. The dose-modulatedirradiating system as set forth in claim 1, wherein the electrostaticcontrol electrode includes an electrostatic grid with grid electrodesarranged for steering the electron beam responsive to an applieddifferential potential.
 3. The dose-modulated irradiating system as setforth in claim 1, further including: a current-modulating means forapplying a time-varying filament current through the filament; and acontrol means for controlling the biasing means and thecurrent-modulating means to produce a selected time varying intensity ofthe electron beam.
 4. The dose-modulated irradiating system as set forthin claim 3, wherein the control means concurrently invokes the biasingmeans and the current-modulating means to vary the filament currentsimultaneous with the time-varying electrical bias on the electrostaticcontrol electrode cooperatively producing the selected time varyingintensity of the electron beam.
 5. The dose-modulated irradiating systemas set forth in claim 1, further including: a rotating gantry on whichthe x-ray tube is disposed, the rotating gantry defining an examinationregion into which the x-ray tube transmits an x-ray beam; atwo-dimensional x-ray detector arranged across the examination regionfrom the x-ray tube that measures a spatially-varying intensity of thex-ray beam after the x-ray beam passes through the examination region;and a processor that reconstructs a computed tomographic image of animaging subject disposed in the examination region based on thespatially-varying intensity of the x-ray beam measured by the x-raydetector at a plurality of positions of the x-ray source.
 6. Thedose-modulated irradiating system as set forth in claim 5, wherein theelectrostatic control electrode includes an electrostatic grid with gridelectrodes arranged about the filament, the dose-modulated irradiatingsystem further including: a second biasing means for applying a switcheddifference electrical bias to the grid electrodes to wobble the electronbeam between alternating focal spots.
 7. The dose-modulated irradiatingsystem as set forth in claim 5, further including: a filament currentcontroller that applies a time-varying filament current through thefilament; and a controller that controls the biasing means and thefilament current controller to produce a selected time varying radiationdosage applied to the imaging subject.
 8. The dose-modulated irradiatingsystem as set forth in claim 5, further including: a filament currentcontroller that applies a time-varying filament current through thefilament; a feedback element that computes a control signalcorresponding to a rate of radiation delivered to the imaging subjectbased on the spatially-varying intensity of the x-ray beam measured bythe x-ray detector; and a controller that controls the biasing means andthe filament current controller to produce a substantially constantcontrol signal.
 9. The dose-modulated irradiating system as set forth inclaim 1, wherein the electrostatic control electrode includes pairedgrid electrodes arranged on opposite sides of the filament, and theelectrostatic control modulator additionally applies a switcheddifferential electrical bias component applied to the grid electrodesthat causes a wobbling of the electron beam.
 10. The dose-modulatedirradiating system as set forth in claim 1, wherein the electrostaticcontrol electrode includes a Wehnelt cylinder.
 11. The dose-modulatedirradiating system as set forth in claim 10, further including: anelectromagnetic deflector that selectively deflects the electron beam.12. The dose-modulated irradiating system as set forth in claim 1,further including: a computed tomography imaging scanner on which thecathode, the anode, and the electrostatic control electrode are mountedas a unitary x-ray tube unit.
 13. A method for dose-modulating an outputof an x-ray tube that includes a cathode having a filament thatgenerates electrons which are focused into a beam, an anode thatgenerates x-rays responsive to the electron beam, and an electrostaticcontrol electrode that electrostatically adjusts an intensity of theelectron beam, the method including: applying a time-varying electricalbias to the electrostatic control electrode to produce a firsttime-varying intensity modulation of the electron beam.
 14. The methodas set forth in claim 13, further including: simultaneously with theapplying of a time varying electrical bias, applying a time-varyingfilament current to produce a second time-varying intensity modulationof the electron beam, the first and second time-varying intensitymodulations cascading to enhance a dynamic range over which theintensity of the electron beam is modulated.
 15. The method as set forthin claim 14, wherein a ratio of a maximum x-ray beam intensity to aminimum x-ray beam intensity during the time varying intensitymodulation is at least 8:1.
 16. The method as set forth in claim 13,further including: synchronizing the applying of the time-varyingelectrical bias with a rotation of a rotating gantry of a computedtomography apparatus on which the x-ray tube is arranged.
 17. The methodas set forth in claim 13, wherein the x-ray tube is a radiation sourcecomponent of a computed tomography imaging scanner, the method furtherincluding: imaging an imaging subject using the computed tomographyimaging scanner; during the imaging, measuring x-ray intensities usingan x-ray detector component of the computed tomography imaging scanner;estimating a temporally varying radiation dose delivery rate of x-rayradiation delivered to the imaging subject during the imaging based onthe measured x-ray intensities; and controlling the applying of thetime-varying electrical bias during the imaging based on the estimatedtemporally varying radiation dose delivery rate.
 18. The method as setforth in claim 17, wherein the controlling step controls the applying ofthe time-varying electrical bias to maintain a selected generallyconstant radiation dose delivery rate.
 19. The method as set forth inclaim 13, wherein the x-ray tube is a radiation source component of acomputed tomography imaging scanner, the method further including:imaging an imaging subject using the computed tomography imagingscanner, the applying of the time-varying electrical bias to theelectrostatic control electrode being performed during the imaging toprovide modulation of a radiation delivery rate.
 20. The method as setforth in claim 19, further including: controlling a filament current ofthe cathode to produce a second time-varying intensity modulation of theelectron beam, the first and second time-varying intensity modulationsof the electron beam being temporally coordinated to provide themodulation of the radiation delivery rate.
 21. The method as set forthin claim 20, wherein the electrostatic control electrode includes anelectrostatic grid with grid electrodes arranged about the filament, themethod further including: applying a switched differential electricalbias to the grid electrodes concurrently with the applying of thetime-varying electrical bias to wobble the electron beam.
 22. The methodas set forth in claim 13, wherein the time varying electrical biasapplied to the electrostatic control electrode is an analog time varyingelectrical bias.